This invention concerns wastewater treatment, particularly membrane separators or MBRs (membrane bioreactors) submerged in mixed liquor to remove permeate and concentrate the mixed liquor.
More specifically, the invention is concerned with the use of a single type of air diffuser for MBRs and optionally for other wastewater treatment applications, with adjustment to provide coarse, mid-size or fine bubbles and to arrive at a targeted mix of such bubbles appropriate for the particular application.
Air diffusers have been used in wastewater treatment for two primary purposes: supplying oxygen to the biological processes, and supplying mixing energy to basins. When used for supplying oxygen, fine bubble diffusers are usually used. The fine bubbles have a greater ability to supply oxygen because of their higher ratio of bubble area to gas volume, as well as their slower rise velocity in liquids. The slow rise velocity, however, renders the fine bubbles less efficient at mixing. More energy is expended for fine bubbles to provide a given mixing energy as compared to larger bubbles.
When used for supplying mixing energy, coarse or mid-size bubble diffusers are typically chosen. The coarse bubbles are more efficient at mixing (electric cost per unit volume of basin) due to their higher rise velocity and lower pressure drop across the diffuser.
When used in a submerged membrane bioreactor (MBR, sometimes referred to as submerged membrane unit, SMU), diffusers are used primarily for facilitating the movement of mixed liquor past the membrane surfaces (a pumping/mixing action), while simultaneously cleaning and scouring the surfaces of the membranes with the mechanical energy of the combined air, liquid and solids movement. Oxygen transfer from the bubbles is a secondary goal, and in some applications, it can be a detriment to the overall treatment system design.
It would be desirable if a single configuration of diffuser could be used for different applications, whether to maximize the supplying of oxygen to mixed liquor or to maximize mixing and scouring action. Further, it would be desirable to have diffusers that could be adjusted to suit these different objectives.